285 
fxus63 kfsd 050210 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
909 PM CDT Friday Jul 4 2008 


Discussion... 
ac field gradually dissipating across lower MO valley...and most of the 
remainder of the night should be quiet as low level gradient slowly 
increases. Small elevated cap noted on evening soundings...and this 
also represented in forecast radiosonde observations around 12z. Not sure if will be able to 
get any isolated convection to form with lack of stronger lift forcing 
to overcome this little setback. Otherwise...some tweeking to trends 
about all that needed. Enjoy the fireworks. 


&& 


Aviation... 
VFR ufn. 


&& 


Previous discussion... 
weak short wave continues to move southeast along the MO river. Isolated altocumulus castellanus/rw- persists 
with this feature and will need to continue this toward 00z early this 
evening. This should weaken and dissipate during the evening...but some middle 
clouds will persist across that area through the evening hours. Later 
tonight...another weak short wave drops southeastward across County Warning Area with some middle level 
instability S of i90 along the i29 corridor late tonight. Could again 
see some altocumulus castellanus precipitation along that axis after 08z as low level jet kicks in a 
bit and will introduce isolated trw-. Low level southerly flow should help hold 
temperatures up into the lower to middle 60s. 


Continued the isolated thunderstorms and rain Saturday morning in a north to S elongated band 
near the I 29 corridor. Thhis is based on a strong low level jet 
continuing/warm air advection and where a weak area of cin exists coupled with a 
persisting vorticity filament shown on both the NAM and GFS. After that 
by afternoon...the fsd forecast area looks largely capped off so left things 
dry. Based on 850 mb temperatures...Saturday should be a warm day but nothing 
unusually hot for this time of year. 


Saturday night...another strong short wave develops and moves eastward toward 
this area. Quite a difference continues to exist between the NAM and GFS 
in degree of warmth of the 700 mb temperatures...especially at 06z Sunday. This short wave 
flattens the flow and is over Minnesota by 12z Sunday. In the evening... 
believe most of the forecast area will be capped off ahead of the wave...so only 
kept our small probability of precipitation going in our far west. But overnight and late 
Sat tonight...the height falls are fairly decent along with a cooling 
of the middle level temperatures. This is coupled with more very strong warm air advection in 
our southeastern quarter of our forecast area. Therefore placed the highest probability of precipitation in 
parts of our northern and western forecast area where the coolest middle level temperatures exist 
and strongest middle level -div q. But also have slight chance probability of precipitation elsewhere 
with a small threat of warm air advection/altocumulus castellanus ts. 


On Sunday...began trimming probability of precipitation from our northwest. The qg forcing drifts off 
to the east and S from east central South Dakota. And a huge discrepancy still exits 
between the NAM and GFS in how far south it brings the surface boundary 
behind the short wave. By 00z Monday...decided to compromise with the surface 
boundary placement which was going with the Canadian Gem. Therefore 
wanted to keep the probability of precipitation near the anticipated surface boundary and upper 
dynamics. Maximum temperatures are also a bit tricky on Sunday...because if the 
NAM pans out with its more aggressive surface boundary...our northern forecast area will 
not warm up into the middle 80s. This is Worth watching on future 
shifts. 


Sunday night...models are in good agreement now in slowing down the 
mesoscale convective system and next short wave with it. For now...kept our probability of precipitation near the surface 
boundary across our south...and in our west late Sunday night with 
this next advancing short wave and mesoscale convective system. But it was very tempting to 
eliminate the probability of precipitation late Sunday night where I put in a slight chance as I 
believe nothing will happen in that area. But for now just decided to 
reduce the probability of precipitation through our east. If the models still show a slowing 
short wave tonight...the night shift may want to eliminate probability of precipitation in our east 
after midnight Sunday night. 


Then on Monday...it looks to be a potentially stormy day with all 
12z models in agreement with a vigorous short wave and upper trough passage. 
Raised probability of precipitation to just under likely in some locations where very good 
dynamics exists. And also went a full category or two below the latest 
guidance readings for maximum temperatures in anticipation of cloud cover and 
precipitation. This wave then exits this area Monday evening. 


In the extended Tuesday through Friday...a pretty good northwest flow aloft 
exists on Tuesday and Tuesday night as a strong upper trough moves down from S 
central Canada on the heels of mondays disturbance. Therefore cooled 
off our temperatures and added a small pop through the northern half of our forecast area with 
very cool air aloft possibly producing some instability ts. After 
that...a large degree of discrepancy exists between the various medr 
models Wednesday through Friday in what kind of overall flow to give this area. 
Some model runs show a flat flow with the polar jet to our north 
which is the warmest solution like the 00z GFS. Other model runs 
like the 00z European model (ecmwf)/06z GFS/12z GFS keep a northwest flow aloft going all 
week. While the latest 12z European model (ecmwf) now carves a large upper trough by 
weeks end into the western Continental U.S.. for now...kept a small degree of northwest 
flow going all week which will likely give US a bit cooler temperatures then 
what I had in yesterday. Also...there will likely be some ts chances 
somewhere but exactly where and when is hard to pin down. So just 
left dry for now after Tuesday. /Mjf 


&& 


Fsd watches/warnings/advisories... 


Iowa...none. 
Minnesota...none. 
NE...none. 
South Dakota...none. 


&& 


$$ 


Chapman/ryrholm